Glass Breakage in Insulating Glazing Units in Spandrel Assemblies

Glass Breakage in Insulating Glazing
Units in Spandrel Assemblies
George Torok, CET, BSS
Morrison Hershfield Corporation
200-2932 Baseline Road, Ottawa, ON Canada K2H 1B1
613-739-2910 • gtorok@morrisonhershfield.com
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George Torok is a façade specialist in the façade engineering team of his firm’s Ottawa,
Ontario, office. He provides specialist consulting services to architects, building owners,
developers and builders, and façade system manufacturers across Canada and the U.S. He
has over 30 years of experience in new building enclosure design and construction and existing
building performance failure investigation, rehabilitation, and renewal. His specialty is
fenestration systems, including windows, doors, skylights, curtainwalls, window walls, sloped
glazing, and glazed architectural structures. He is a past president of the Ontario Building
Envelope Council and the Building Envelope Council Ottawa Region, and he is a director of
the Building Science Specialist Board.
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ABSTRACT
SPEAKER
Curtainwall and window wall systems today commonly use insulating glass units (IGUs) for vision glazing and for
spandrel panel cladding. A risk for IGUs in insulated spandrels is higher thermal stresses on the glass lites, compared
to older approaches with single glazing. When ceramic enamel (frit) opacifiers are used, there is increased stress from
solar absorption. Thermal stress is generally addressed by heat-treating at least the opacified lite. Multiple incidences
of thermal stress-related fracture of ceramic enameled spandrel glass in North America, Europe, and elsewhere have
shown that ceramic enamel weakens glass, reducing the added benefit of heat treatment, so there remains a risk of
in-service thermal stress breakage. This presentation will give examples of breakage, describe causes, show results
of a test program that shows that ceramic enamel reduces the strength of heat-treated glass, and describe options
to control the potential for in-service breakage. The discussion will be of interest to building owners, designers, and
builders considering the use of curtainwall and window wall systems with opacified glass.
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THE PROBLEM
Curtainwall systems today commonly
use insulating glass units
(IGUs) for vision and spandrel glazing.
Low-emissivity (low-e) coatings
with increasingly lower U-factors
and solar heat gain coefficients are
used to meet modern energy code
requirements. Thermal insulation
is used behind spandrel IGUs to
further control heat loss in spandrels.
To conceal the insulation,
the inboard lite of spandrel IGUs
is often made opaque (opacified) by
applying a ceramic enamel coating
to the back face. Sometimes,
the enameled glass fractures, even
though the glass is heat-treated
during enamel application.
This paper will give examples of
breakage, describe causes, and discuss
options to control the potential
for in-service breakage.
WHAT ARE CERAMIC
ENAMEL COATINGS?
A ceramic enamel coating is,
essentially, a glassy paint fused
onto the surface of a pane of architectural
vision or spandrel glass. It
is applied in liquid form and has
the same fundamental components
as any paint: pigments, vehicle, and various
additives. The pigments (metal oxides)
give color opacity or transparency. The
vehicle or medium is the solvent into which
the pigments and additives are dissolved
for application to the glass surface. The
main component of the vehicle by weight is
ground-up glass (frit), which acts as a flux
(it reduces the firing or fusing temperature
of the compound), a binder (by melting and
fusing with the glass surface, it holds other
non-volatile components to the glass), and a
durability enhancer, providing resistance to
chemical and physical wear. Additives can
perform many functions, depending on the
needs of the finished product.
There are many ways to apply ceramic
enamel to glass: screen printing (the traditional
approach for patterns and repetitive
images), ink jet (the modern approach for
both patterns and complex imagery) and
roller, spray, and curtain coating (for uniform
or “full-flood” coverage). After application,
the enamel is dried (the solvent
evaporates), and then the coated glass is
heat-treated (heat-strengthened [HS] or
fully tempered (FT]), which fuses the coating
to the glass surface (as well as imparting
resistance to thermal stress or impact
to the glass substrate).
OBSERVED BREAKAGE OF
ARCHITECTURAL CERAMIC
ENAMELED GLASS
When glass coated with ceramic enamel
breaks, the breakage pattern is determined
by the type of stress that occurred
(thermal, mechanical) and the nature of
the glass substrate (HS, FT). Studies of
ceramic enamel-coated glass that cracked
in service indicate that thermal stress
is usually the cause, although mechanical
stress may be a contributing factor.
Thermal stress cracks usually begin at the
perimeter and progress inward toward the
center-of-glass region. With repeated application
of stress, cracks may branch, continue
to another edge, and/or return to the
same edge. HS glass breaks into pieces of
various sizes—often quite large—characterized
by flowing/curving cracks. The irregular
shapes of broken HS glass shards tend
to lock together, so for the most part, the
broken glass stays in place (Figures 1 and
2). FT glass shatters into small pieces and
usually falls out of the IGU or fenestration
product frame.
Glass Breakage in Insulating Glazing
Units in Spandrel Assemblies
Figure 1 – Fractured heat-strengthened glass in a single-glazed curtainwall spandrel
panel. The crack likely started at the bottom, extending into the center-of-glass region,
before branching and continuing across the unit to the top.
Close-up examination typically shows the crack origin is inward of
the glass edge, at the enamel-coated face of the glass (Figures 3 and
4). The origin of the crack at the ceramic enameled surface indicates a
damaging effect of the enamel to the glass surface. The specific nature
of the damage is uncertain, but it may be related to intrusion of particles
of frit into the glass surface, acting as stress concentrators.
Research is ongoing.
STUDIES CONFIRMING THE EFFECT OF
CERAMIC ENAMEL ON GLASS STRENGTH
Breakage of this type has been observed in architectural
spandrel glass with complete coverage of one of
the flat faces (“flood coat”). It has also been observed in
architectural vision glass with enamel applied in a pattern,
such as for privacy or solar control and in automotive
vision glass with ceramic enamel masking around
the perimeter.
Test methods used to assess the weakening of glass
with ceramic enamel coating include:
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Figure 2 – Fractured heatstrengthened
glass in an inboard
pane of glass in an IGU of a
curtainwall spandrel panel. The
crack started at the upper left corner,
to the right of the temporary restraint
(dutchie) to the curtainwall frame
behind, indicating a combination of
thermal stress and mechanical stress
contributed to breakage.
Figure 3 – View of the back side of the
spandrel IGU in Figure 2, after it was
removed from the curtainwall. The crack split
toward the center-of-glass area, as seen in
Figure 2 and also to the edge of the inboard
pane, indicating the origin of the crack was
inward from the edge.
Figure 4 – View of the crack face, exposed
after glass on one side of the crack was
removed. Visible are features typical of a
crack origin (mirror, mist, and hackle),1
which indicate the crack origin is at the top,
at the flat face of the glass with the ceramic
enamel coating, and with the fracture
radiating away into the thickness of the
glass, most clearly indicated by the ridges in
the hackle. The origin is located within the
depth of the perimeter spacer and sealants.
• Uniform load deflection: ASTM
E998, Standard Test Method for
Structural Performance of Glass in
Windows, Curtain Walls, and Doors
Under the Influence of Uniform Static
Loads by Nondestructive Method
• Four-point bending: ASTM C1161,
Standard Test Method for Flexural
Strength of Advanced Ceramics at
Ambient Temperature
• Impact: GANA LD 100-06,
Standard Test Method for Ball Drop
Impact of Laminated Architectural
Flat Glass; ASTM F3006, Standard
Specification for Ball Drop
Impact Resistance of Laminated
Architectural Flat Glazing; and UN
Global Technical Regulation No. 6,
Safety Glazing Materials for Motor
Vehicle and Motor Vehicle Equipment
These methods assess load capacity in
bending, either through slow application
of force (uniform load deflection) by fourpoint
bending tests or rapid application
by impact by ball drop tests. These test
methods do not measure thermal stress
capacity directly, but since thermal and
mechanical stresses have the same effect
on glass—increasing tensile stress—these
bending tests can be used as surrogate test
methods. Examples of studies demonstrating
that ceramic enamel coatings weaken
HS and FT glass, assessed by
using these test methods, are
given below.
UNIFORM LOAD
TESTING (ASTM E998)
The four-point testing
method involves sealing a glass
pane against a chamber that is
pressurized or depressurized,
then measuring and observing
the effects on the glass. In the
study from which the graph in
Figure 5 is taken, the chamber
was evacuated until the glass
samples broke. All samples
were heat-treated. The ceramic-
enamel-coated side faced
into the chamber, so it was put
into tension. The tests revealed
that the failure load decreased
as ceramic enamel frit coverage
increased.
The graph is limited to
the probability of breakage
up to 0.010, or 10
panes in 1,000, so
it doesn’t show the
effect of breakage
to failure (i.e., probability
of breakage
of 1.0). This is to
more clearly show
behavior in the
region of most interest
to structural
design of glass for
buildings which is
typically limited to a
maximum probability
of eight breaks
in 1,000.
Numeric analysis
reveals that, at
the 8/1,000 probability
level, the
bending strength of FT glass with partial
coverage of ceramic enamel coating (dots,
lines, holes) is about 80–85% of uncoated
HS glass. The bending strength of FT with
full coverage is approximately 63% that of
the uncoated HS.
FOUR-POINT BENDING
(ASTM C1161)
This test method involves breaking
samples of glass in a four-point bending
machine, in which the samples bridge
across two static cylindrical pins, and a
platen with two other similar pins spaced
closer together is moved downward on the
sample until it breaks. The break load is
measured. Ceramic-enamel-coated glass
is set face down so the enamelled face is
put into tension. Similar to Figure 5, the
graph compares probability of breakage
against load, to highlight the effect in the
8/1,000 range used for structural design
of glass. Four types of glass are compared:
uncoated and 100% ceramic-enamel-coated
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When glass coated
with ceramic enamel breaks,
the breakage pattern is
determined by the type of
stress that occurred (thermal,
mechanical) and the nature
of the glass substrate
(heat-strengthened [HT] or fully tempered [FT]).
Figure 5 – Probability of breakage vs. three-second equivalent failure load. This graph shows
the results of uniform load bending to failure, for heat-treated glass without ceramic enamel
coating (clear) and glass with different extents of ceramic enamel coating coverage (dots =
40%, lines = 50%, holes = 60%, full-flood = 100%). The horizontal line at 0.008 probability of
breakage represents the normal allowable breakage limit of eight breaks in 1,000 for glass
design in buildings (from Bergers, Natividad, Morse, and Norville, 2016).
heat-strengthened glass, and uncoated and 100% ceramic-enamel-coated FT glass.
The graph shows that the load at which breakage occurs, in general, is reduced for both ceramic-enamel-coated, HS and FT glass, compared to uncoated versions of the same glass type.
In a 2018 paper, Barry—the author of the paper from which Figure 6 is taken—summarizes the results of 10 studies of uniform load tests (i.e., ASTM E998) and four-point bending tests (ASTM C1161) for HS glass with and without ceramic-enamel coating. Design strength (i.e., at 8/1000th probability of breakage level) reductions range from 2% to 60%, with an average of about 31%.
BALL DROP (GANA LD-100, ASTM F3006, UN GLOBAL TECHNICAL REGULATION NO. 6)
In these standards, a similar test method is used which involves placing samples of glass on a four-sided frame and dropping a steel ball onto the sample. The height at which the ball is dropped is increased in a step-wise fashion until the sample breaks. The drop height at breakage is recorded. Ceramic-enamel-coated glass is set face down so the enamelled face is put into tension by the impact of the steel ball on the opposite (upward-facing) side. Figure 7 shows results of a test program that the author’s firm participated in. Similar to Figure 5, the graph compares probability of breakage against load, to highlight the effect in the 8/1000 range used for structural design of glass. Four types of glass are compared: uncoated and 100% ceramic-enamel-coated, HS glass, and uncoated and 100% ceramic-enamel-coated FT glass.
The graph shows that drop height (and, therefore, impact load) is dramatically reduced when tempered glass is fully coated with ceramic enamel, and that drop height (and impact load) increases when a silicone coating is applied (specific products tested were Opaci-coat 300 and Opaci-coat 500 by ICD).
Results of a study in the automotive sector, using a similar test method (UN Global Technical Regulation No. 6, Safety Glazing Materials for Motor Vehicle and Motor Vehicles Equipment), allow us to compare the effect of ceramic coating on FT glass with regular annealed glass. The study addressed breakage of automobile sunroofs with ceramic enamel masking around the perimeter to conceal adhesive bonding to the vehicle frame. The study compared uncoated annealed glass, uncoated FT glass, and perimeter-coated, ceramic-enameled FT glass. The results of this test
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Figure 6 – Probability of breakage vs. three-second equivalent stress load. This graph shows the results of four-point bending until breakage, for uncoated and 100% ceramic-enamel-coated, HS glass, and uncoated and 100% ceramic-enamel-coated FT glass (from Barry, 2015).
Figure 7 – Break height during ball-drop tests of FT glass, uncoated (left, grey bar), coated with two versions of silicone coating (middle orange and red bars), and ceramic enamel (right, black bar) (from Vockler, Krytenberg, Norville, Blanchet, Swanson, Barry, Carbary, Hoffman, Torok, and Fronsoe, 2017).
show that the impact resistance of FT glass with a perimeter ceramic-enamel coating is reduced to less than that of uncoated, annealed glass (Figure 8).
In summary, the studies reveal that:
• Ceramic enamel coating reduces bending strength and impact resistance of HS and FT glass.
• Weakening occurs with full coverage and partial, patterned coverage. Roughly, the reduction in load capacity is proportional to the extent of coverage of the glass surface by ceramic enamel.
• If HS glass is needed for improved thermal stress and/or mechanical stress, given the strength reduction that occurs with ceramic enameling, consideration should be given to using FT glass instead; but beware of unintended consequences (see Recommended Practice below).
GUIDANCE FOR THE DESIGN PROFESSIONAL
In Europe, the governing standard for FT glass (known as “toughened” glass) is EN 12150, Glass in Building – Thermally Toughened Soda Lime Silicate Safety Glass. That standard requires a load reduction of 38% for ceramic enamelled glass, which is within the broad range of capacity reductions revealed in the studies discussed previously.
Despite the approach taken in Europe and a growing body of empirical evidence and the results of studies just described, discounting the thermal stress and/or bending stress capacity of glass during the design of fenestration systems remains controversial in North America. In late 2018, the committee that oversees ASTM E1300, Standard Practice for Determining Load Resistance of Glass in Buildings, began to study the issue, with a goal to revise the standard to include provisions for addressing the strength of ceramic-enameled architectural glass. That work is ongoing. In the meantime, how should a design professional proceed? Three approaches are outlined below.
1. Consider using other coatings.
• If simple full-flood coverage is needed, two alternatives are liquid-applied silicone coating or a polyester film (“scrim”) opacifier. The study by Vockler et al., cited earlier in Figure 7, shows that liquid-applied silicone opacifiers available from one manufacturer do not adversely affect bending or impact performance of tempered glass as does ceramic enamel. The author is not aware of incidences of polyester film opacifiers weakening heat-treated glass.
• If patterns, photographic images, or artwork images are desired, laminated glass with the images printed on the interlayer material is a possibility. However, laminated glass is normally made with regular annealed glass and so would not resist the elevated levels of thermal stress in spandrel panel glazing. It is possible to laminate with HS or FT glass to provide the necessary thermal stress resistance, but in turn, there are some fabrication challenges to address (matching roller wave and edge lift distortions, for example) and the interlayer material must then be capable of resisting high temperatures that develop in spandrel glazing. The assistance of a heat treatment glass processor and interlayer manufacturer should be obtained when considering this approach.
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Figure 8 – Break height during ball drop tests for uncoated annealed (“original”) glass, uncoated FT (“toughened”) glass, and FT glass with a full ceramic-enamel coating (“ceramic-printed toughened glass”) (from Kwang-bum, Hae-boung, Ho-soon, Sang-woo, and Young-sam, 2015).
Despite the approach taken in
Europe and a growing body of
empirical evidence and the results
of studies just described, discounting the thermal stress and/or bending stress
capacity of glass during the design of fenestration systems remains
controversial in North America.
2. Design for reduced load capacity.
• Reduce the bending stress load capacity of the glass at the design stage, following European practice (i.e., discount bending strength by 38%).
• If load capacity is reduced, compensating actions may be needed. For example, for mechanical stress (wind load), some lost load capacity could be restored by increasing glass thickness. However, increasing glass thickness does not improve thermal stress resistance.
• If HS glass is required for thermal stress or wind load resistance, increase to FT. However, this introduces a risk of spontaneous breakage due to nickel sulphide (NiS) inclusions. It is possible to control—but not prevent—this risk by further processing FT glass using the European heat soak test, EN 14179-1, Glass in Building: Heat-Soaked Thermally Toughened Soda Lime Silicate Safety Glass.
3. Reduce applied loads.
• The key to controlling thermal stress in fenestration glazing is to reduce center-of-glass to edge-of-glass temperature differences. That can be achieved by reducing the temperature at the center of the spandrel glazing, increasing the temperature at the edge of the glazing, or a combination of the two.
• Center-of-glass temperature rise can be controlled by the use of reflective coatings or pyrolytic low-e coatings for single glazing, and sputtered low-e coatings in sealed IGUs. However, it must be kept in mind that an IGU with low-e-coated glass restricts the loss of heat gain from solar exposure back to the exterior, and insulation behind the spandrel glazing restricts heat loss to the interior, so the spandrel glass center-of-glass temperature can still become quite hot.
• Some curtainwall systems include vents at the top and bottom of the spandrel cavity in an attempt to allow solar heat gain to escape to the exterior. However, research conducted by the author’s firm has shown this is not an effective approach (Figure 9).
• Edge-of-glass temperature can be increased by reducing thermal bridging through the fenestration product frame and, if used, through the IGU spacer and sealants. A frame with higher-performing thermal breaks and an IGU with a warm-edge spacer could be used.
A growing body of experience and research shows that ceramic-enamel coatings on heat-treated architectural glass reduce thermal stress, bending stress, and impact resistance capacities. Care should be taken when considering ceramic-enamel coatings on architectural glass subject to such stresses, especially where one or more may be present at elevated levels, which is often the case in spandrel panel glazing. Some guidance is provided in this paper, based on experience and on European practice to reduce the risk of breakage. When in doubt, seek the assistance of a design professional.
REFERENCES
C.J. Barry and H.S. Norville. “Unexpected Breakage in Ceramic Enameled (Frit) HS IG Spandrels.” IGMA Winter Conference. Fort Lauderdale, FL. 2015.
M. Bergers, K. Natividad, S.M. Morse, and H.S. Norville. “Full-Scale Tests
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Figure 9 – Center-of-glass to edge-of-glass temperature differentials for spandrel IGU with opacifier on surface four. At left, single-glazed spandrel glass; and at right, double-glazed sealed IGU spandrel glazing. Three conditions were studied: glazing sealed on all four sides to the curtainwall frame, drained through the sill, and drained through the sill and vented through the head. Little difference was found among the options (from Schwartz, Roppel, Hoffman, and Norris, 2018).
of Heat Strengthened Glass with Ceramic Frit.” Challenging Glass. 2016.
L. Kwang-bum, K. Hae-boung, A. Ho-soon, J. Sang-woo, and S. Young-sam. “A Study on Toughened Glass Used for Vehicles and its Testing Methods.” 24th International Technical Conference on the Enhanced Safety of Vehicles. Gothenburg, Sweden. 2015.
G.D. Quinn. Fractography of Ceramics and Glasses. National Institute of Standards and Technology (NIST) Special Publication 960-16e2. Washington, D.C. 2016.
J. Schwartz, P. Roppel, S. Hoffman, and N. Norris. “Glazed Spandrels: Quantifying the Benefit of Venting to Minimize Risk of Glass Breakage.” Façade Tectonics World Congress. Los Angeles, CA. 2018.
K.L. Vockler, T.P. Krytenberg, H.S. Norville, S. Blanchet, J.W. Swanson, C.J. Barry, L.D. Carbary, S.P. Hoffman, G.R. Torok, and C.S. Fronsoe. “Silicone Opacifiers for Spandrel Glass Applications: Risk Mitigation in Thermal Stress.” Glass Performance Days. Tampere, Finland. 2017.
B. Weller and P. Krampe. “The Effect of Enamel on Glass.” Glass Performance Days. Tampere, Finland. 2013.
FOOTNOTE
1. The mirror, mist, and hackle derive their names from their appearance, when a crack is separated and the edges examined. The mirror is smooth and reflective to the naked eye, the mist appears similar to condensation on a mirror, and the hackle is roughly textured like broken shards of glass oriented in the direction of crack movement. Each feature represents stages in the progression of the crack from explosive release of pent-up stress (the mirror) and increasing turbulence as the advancing energy dissipates and spreads through the glass (mist and hackle). These three features identify the origin of a crack and can be analyzed to reveal the type and amount of stress present at the moment of fracture, from which the cause of fracture can be determined.
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